In a conventional ignition apparatus, as shown in FIG. 8, a spark plug 30 mounted on a spark-ignited engine has a center electrode 31 and a ground electrode 32. Electric discharge is generated between the electrodes 31 and 32 normally as indicated by SP1, which takes the shortest path. Recently however turbulent flow such as tumble flow or swirl flow is generated in a combustion chamber of a lean-burn engine to improve combustion. In such an engine, which generates the turbulent flow, air flows as an air stream F at high speeds within a combustion chamber. The air stream F changes the discharge in the form of SP1 to a discharge in a form of SP2, which has a longer discharge path.
When the air flows at high speeds, the discharge is blown out or extinguished once and immediately thereafter the discharge restarts in the shortest distance (path indicated as SP1) between the electrodes 31 and 32. Even when the discharge is generated again, it may be blown out again by the air stream F. Thus repetition of the blowout and the discharge arises and causes exhaustion of the electrodes 31 and 32 (plug exhaustion) much faster than usual.
For example, the blowout is not caused in a capacitive discharge period (short period near t3 in FIG. 2) because the secondary current is sufficiently large. The blowout arises in an inductive discharge period (period t3 to t4 in FIG. 2), in which the secondary current gradually decreases.
Some ignition apparatuses disclosed in JP 2001-193622A and JP 2000-345951A, for example, counter the plug exhaustion caused by the repetition of discharges as follows. Specifically, a primary current is supplied to terminate forcefully the discharge at an end of a discharge period from a start of the discharge. The discharge period is set in accordance with operating conditions of the internal combustion engine. Thus, a period, in which the blowout is likely to arise, can be eliminated in the inductive discharge period t3 to t4, in which the secondary current is small. As a result, the repetition of the discharge can be avoided and the exhaustion of plugs can be countered.
However, the speed of air stream F, which causes the blowout, differs due to variation in the angle of mounting of the spark plug on the engine. The air stream condition in the cylinder is not stable and varies from time to time. It is therefore very difficult to determine whether the speed of air stream will cause the blowout in each engine. It is therefore very difficult in the conventional ignition apparatuses to set a discharge period to the most optimum value in correspondence to the air stream condition.
For this reason, if the discharge period is set to be excessively short in spite of low possibility of blowout for example, misfire may be caused due to insufficiency of the discharge period. This misfire becomes critical in an operating condition, in which ignitability is poor. If the discharge period is set to be excessively long in spite of high possibility of blowout, it becomes impossible to avoid the repetition of discharge.